Baking treatment of lithographic printing plate

ABSTRACT

A method of processing a lithographic printing plate which comprises the steps of exposing, developing and baking the plate, in which a pre-baking liquid is applied to the plate prior to the baking step, and which is not tacky, has a long storage life, and does not require rinsing with water after baking. The pre-baking liquid includes one or more of a sulphosuccinamate or sulphosuccinate and a type and present in an amount sufficient to form a non-water soluble baked coating on the developed plate which acts as an effective barrier to contaminating material tending to come into contact with the developed areas of the plate during the baking step, and which has no adverse reaction with the image and non-image areas and allows the plate to be baked, and thereby to improve the durability of the processed plate for use on a press. The baked coating is removed subsequently by an alkaline solution.

This invention relates to a baking treatment of a lithographic printingplate.

It is well known that the inclusion of a baking stage in a lithographicprinting plate treatment process increases the "run length" of the plateon a press very considerably. It is therefore usual for lithographicplate processing equipment suppliers to include a plate baking oven intheir range of equipment, and which bakes the plate to produce a durableink-receptive image area on the processed lithographic plate.

In the treatment of a lithographic printing plate, it is first exposedand then developed by hand or by the use of a processor to reveal theunderlying substrate in the non-image area, prior to coating of theplate with a pre-baking solution. The plate then passes through thebaking oven where it is subjected to a sufficient level of heat toconvert the image area into a more durable form.

A number of baking solutions are commercially available, includingUltratherm and Thermotect, (available from Horsell Graphic Industriesand Howson-Algraphy respectively) and these give generally satisfactoryresults, and which are of generally similar character in that they bothform a water soluble layer on the processed plate.

The purpose of the baking solution is to provide temporary shielding ofthe non-image areas from contact with contaminants, during the bakingstage, while enabling the image areas to be baked and thereby improvetheir durability in use on the press. Therefore, the baking solution,which forms a water soluble layer prior to baking, is believed to form aphysical barrier to any contaminating material present e.g. in the ovenor emanating from the image areas, but it must also be such that it doesnot adversely effect the image by dissolving it, or rendering imageareas non ink receptive, or non-image areas ink receptive. Also, thewater soluble layer is readily removable after the baking process,without the image and non-image areas being adversely affected. The useof water soluble pre-baking solutions forms the subject of, and isdisclosed in more detail in UK patent 1513368.

When water soluble pre-baking solutions are employed, includingparticularly Ultratherm, there are serious problems if there is likelyto be a long delay between issuing from the baking oven and use of thetreated plate on the press. When only a short delay is anticipated, thebaked plates can be taken to the press without any further treatmentbeing required. However, if a long delay is likely, then it is usuallynecessary to apply three further steps in the processing of the platesafter the baking step, including a rinsing step, application of apost-baking (storage gum) and a final drying step. After the rinsingstep. Ultrafin or Unifin (the trade marks of Horsell Graphic Industriesand Howson Algraphy respectively) is usually employed in the post-bakingstage, but this may give rise to certain problems as set out later.

If these further steps are not included, and there is a lengthy delaybetween issue from the baking oven and use on the press, then thefollowing technical problems can arise:

(i) sensitivity of the background is created,

(ii) in humid conditions where plates are stacked they are liable tostick together,

(iii) "blinding" can occur on the image area--it will not accept ink onimage areas without some form of image cleaning.

However, even with the further steps included, while this does solve theproblems of (a) clear background, (b) blinding, and (c) storage life,the use of these specifically available storage gums render the plates"tacky" so that they are awkward to handle and liable to pick up dirt,and can be difficult to transport through automatic conveyors containedwithin plate bending equipment.

The invention therefore seeks to enable an improved treatment ofpre-sensitised printing plates in which a baking stage is required, byusing a pre-baking solution which does not require a post-baking waterrinsing stage.

The invention is therefore based on the use of a different material toform the pre-baking solution, and which does not result in the formationof a water soluble layer which, at least with preferred examples, can besubsequently removed after the baking stage, and which can result in theproduction of a treated lithographic printing plate for use on a presswhich is: (a) not tacky, (b) has a long storage life, and (c) does notrequire rinsing with water after baking.

According to the invention there is provided a method of processing alithographic printing plate which comprises the steps of exposing,developing and baking the plate, in which a pre-baking liquid is appliedto the plate prior to the baking step and includes one or more of asulphosuccinamate or sulphosuccinate of a type and present in an amountsufficient to form a non-water soluble baked coating on the developedplate which acts as an effective barrier to contaminating materialtending to come into contact with the developed areas of the plateduring the baking step, and which has no adverse reaction with the imageand non-image areas and allows the plate to be baked and thereby toimprove the durability of the processed plate for use on a press.

As the material applied prior to baking is no longer water soluble afterthe baking stage, it will be necessary to employ an alkaline post-bakingtreatment.

The pre-baking liquid may include a soluble acid, and preferablyphosphoric acid in a proportion of 0 to 3% by weight.

One particularly suitable type of sulphosuccinamate is made and sold inthe United States under the trade mark AEROSOL 22 by Cyanamid and in theUK under the trade mark Rewopol B2003 by Rewo Chemicals, and preferablyin the form of a 35% solution present in a range 10% to 100% by weight.When phosphoric acid is mixed with AEROSOL 22, preferably it is in therange 0.1% to 1% by weight.

One preferred example of pre-baking solution is set out below:

    ______________________________________                                        Constituents      % by weight                                                                              Activity                                         ______________________________________                                        Water             84.8%      --                                               Aerosol 22 or Rewopol                                                                             15%      5.25%                                            B2003 (35% solution)                                                          Phosphoric Acid    0.2%      0.17%                                            (85% solution)                                                                ______________________________________                                    

This pre-baking liquid is suitable for use with conventional positiveworking lithographic printing plates, and also with plates which undergoconversion e.g. the Gemini process of Horsell Graphic Industries (andsimilar processes of other plate manufacturers). However, in this lattercase, it has been found that unacceptable "cratering" can occur in thelight-sensitive coating if the % of sulphosuccinamates falls below 3.5%by weight of the coating material applied for certain types of plate.However, for some types of plate, with the conversion process, lowerproportions of sulphosuccinamate may be acceptable. Sulphosuccinamateshave the following general structure: ##STR1## and are derivatives ofsuccinamic acid having the general structure: ##STR2##

Another example of pre-baking solution is a sulphosuccinate sold underthe trade mark Rewopol SBD070 by Rewo Chemicals, having the followinggeneral structure: ##STR3## and which is a derivative of succinic acidhaving the general structure: ##STR4##

Preferably, and especially when a long delay is anticipated betweentreatment in the baking step and use on the press the process includestwo further steps, neither of which is a rinsing step. The two furthersteps are the application of a post-baking solution and a drying step.

Details of examples of preferred post-baking solutions are set outbelow:

    ______________________________________                                        Constituents    Proportions by weight                                         ______________________________________                                        VERSICOL* EN9   Minimum 0.5% to maximum 50%                                                   with preferred range 10% to 25%                               Sodium metasilicate                                                                           0.5% to 5% and preferably in                                                  the range 1% to 3%                                            **                                                                            NERVANAID DP    0.05% to 3% and preferred                                                     range 0.1% to 0.8%                                            ______________________________________                                    

When a post-bake solution is employed which includes a film formingmaterial such as VERSICOL, it preferably has the following components:

    ______________________________________                                        Material        % by weight                                                                              Activity                                           ______________________________________                                        Water             80%      --                                                 VERSICOL EN9    17.5%      4.375%                                             (25% solution)                                                                Sodium Metasilicate                                                                             2%          2%                                              NERVANAID DP     0.5%       0.2%                                              (40% solution)                                                                ______________________________________                                    

In the formulation of suitable post-bake solutions, the NERVANAID isjust one example of a suitable water softener, and other materials maybe used. Sodium metasilicate is one example of an alkali for use in thesolution, though other alkalis may be used, such as silicates,phosphates, amines and hydroxides. Concerning VERSICOL, alternativesinclude all types of polyacrylic acids, polymethacrylic acids andpolyvinyl pyrollidone.

The use of sulphosuccinamates in the pre-baking liquid provides abiodegradable material which also exhibits greater environmentalacceptability when compared with other commercially available pre-bakesolutions.

For use in conjunction with reversible positive pre-sensitised plates,as treated with the Horsell Gemini or similar processes, this pre-bakingsolution may also be used to advantage where most other materialsavailable would not work without unacceptable problems.

* VERSICOL--This is the trade mark of Allied Colloids.

** NERVANAID--This is the trade mark of ABM Chemicals.

EXAMPLES

Comparative tests were carried out using examples of pre-baking liquidin a method according to the invention, and other materials, these beingcarried out on a standard Horsell Libra Plate of a size 650 mm×550 mmand a gauge 0.3 mm. This is a particular type of presensitisedlithographic printing plate supplied by Horsell Graphic IndustriesLimited for a number of years.

First of all, the plate was exposed using a 5 kW light source with anapproximate spacing of 1.2 m between the light source and the plate. Theexposure time was sufficient to reveal a clear reference 2 on a stoufferstep wedge using Horsell C1290 positive hand developer for one minute at20° C. Following hand development, using cotton wool, each plate wasrinsed with water and then dried.

The different types of pre-baking solution were then applied by handusing cotton wool to produce an even streak free film. This film wasthen dried using a fan drier. The different types of material used toform the pre-baking solution, and their relative performances, are setout in more detail later.

The plate was then placed in a moving plate oven having the oventemperature set at 575 F. and a linear speed through the oven of 1.75 mper minute. The duration of heating of each plate was approximately 41seconds.

Once the plate has passed through the oven, it was allowed to cool,prior to application of an alkaline post-baking solution, such as any ofthe alkaline post-baking solutions referred to above e.g. sodiummetasilicate. The purpose of this treatment is to remove the bakedcoating derived from the pre-bake solution, along with any contaminatingmaterial present on the plate. The resulting application of the postbaking solution must prevent redepostion of contaminants onto therevealed non-image areas.

The post baking solution was applied by hand for approximately 30seconds at 20° C.

The plate was then rinsed with water to remove the post baking solution,then squeegeed and inked.

The plate was evaluated visually in both the image and non-image areas.Any detrimental attack on the image area would be seen in the solid orhalf tone areas which constitute the image area.

As will be apparent from the detailed example of pre-baking solutionsused, with the exception of sulphosuccinates and sulphosuccinamates, theperformance of the other materials tested was unsatisfactory.

The major problem with most of the materials tested was the inability toprotect the substrate from contamination. A further problem was theformation of craters in the light sensitive coating formed during bakingof the pre-baking solution on the plate. These crates appeared as if thelight sensitive coating had bubbled during heating, and quite often asmall deposit of coating remained in the middle. This problem however isonly observed on plates which have undergone the Horsell GraphicIndustries "Gemini" plate conversion process, or similar process ofother plate manufacturers, or on plates which have been exposed tolights prior to the baking process.

To reduce the level of background (substrate) contamination, smallamounts of acid may be incorporated into the pre-bake solution. Citricacid was used with some surfactants, as it has the advantage of beingwater-soluble, whereas other acids used, such as phosphoric and boricacid became insoluble, and could only be removed on application of thealkaline post baking solution. In the production of positive plates,while sulphosuccinamates and sulphosuccinates provide acceptableperformance in the absence of added acids (such as citric, phosphoric orboric acids), it has been found to be advantageous to add one or moresuch acids to the pre-baking solution.

However, in the case of plates converted by the Gemini process, it hasbeen found to be essential to add acid to the sulphosuccinates to obtaina commercially acceptable result, and in the case of sulphosuccinamatesit is certainly desirable.

The materials tested, their general class, and the results obtained areset out on the table listed below:

    ______________________________________                                        Class    Type                  Result                                         ______________________________________                                        Amphoteric                                                                             Betaine derivatives                                                  Anionic  Alkyl aryl ethern sulphates                                                   Fatty acid soaps                                                              Olefin sulphonates                                                            Sarcosinates                                                                  Sodium butoxyethoxyacetate                                                    Quaternary ammonium compounds                                        Non-ionic                                                                              Acetylenic diols      Poor                                                    Alkylolamides                                                                 Ethoxylated alkylolamides                                                     Ethoxylated alkyl phenois                                                     Ethoxylated castor oil                                                        Ethoxylated fatty acids                                                       Ethylene oxide / propylene                                                    oxide copolymers                                                     Amphoteric                                                                             Alkyl ether hydroxypropyl                                                     sultaines                                                                     Fatty amidoamine sulphates                                                                          Moderate                                       Anionic  Phosphate esters                                                              Sulphated fatty acid esters                                                   Taurates              Fairly good                                             Isethionate                                                                   * Sulphosuccinates    Very good                                               * Sulphosuccinamates                                                 ______________________________________                                         * The sulphosuccinates and the sulphosuccinamates which were tested, and      which provide very good results, were commercially available materials        under the trade mark Rewopol SBD070 and B2003, which at present include       small amounts of solvents (ethanol, propylene glycol or isopropanol) to       assist in the preparation of a stable homogeneous solution for applicatio     onto the plates.                                                              At present levels, these solvents do not have any adverse effect though i     is believed that higher levels might be disadvantageous. However, while       these solvents are useful in the applications of the solutions, they are      not absolutely essential.                                                

We claim:
 1. A method of processing a lithographic printing plate whichcomprises the steps of exposing the plate, developing the plate,applying to the plate a pre-baking liquid including one or more of asulphosuccinamate or sulphosuccinate, of a type and present in an amountsufficient to form a non water-soluble baked coating on the developedplate which acts as an effective barrier to contaminating materialtending to come into contact with the developed areas of the plateduring a subsequent baking step, and baking the plate, wherein thepre-baking liquid has no adverse reaction with the image and non-imageareas of the plate and allows the plate to be baked thereby to improvethe durability of the processed plate for use on a press.
 2. A methodaccording to claim 1, comprising a further step of treating the bakedplate using an alkaline solution in order to remove the baked-on coatingand any contaminants which may be present.
 3. A method according toclaim 1 in which the pre-baking liquid includes an acid.
 4. A methodaccording to claim 3, in which the acid is from the group consisting ofcitric acid, boric acid and phosphoric acid.
 5. A method according toclaim 1, in which said pre-baking liquid includes a sulphosuccinamateused in an amount not less than 3.5% by weight of the applied pre-bakingliquid.